More than 90,000 tons of radioactive waste are stored on-site at nuclear power plants awaiting permanent disposal. Nuclear waste releases unstable forms of uranium and other elements, called radionuclides. Long-term exposure to radionuclides can cause cancer, so environmental safety regulations require that nuclear waste be made inaccessible for at least 1 million years. Thus, researchers are evaluating possible ways to store this waste permanently. 

One proposal is to store nuclear waste thousands of feet below ground in rock formations called deep geological repositories, or DGRs, to seal in radionuclides. Scientists can’t see into the future, so instead they look to the past to determine how well DRGs could work. A team of researchers from Canada recently analyzed uranium deposits that have existed for billions of years as an analog for how nuclear waste behaves deep underground.

Past researchers found that uranium deposits in ideal DGR conditions, with their protective casings intact, will stay contained for more than a billion years. However, these conditions require a stable geological environment. This team instead focused on uranium deposits in non-ideal conditions. They studied the Canadian Kiggavik uranium deposits, which occur in cracked bedrock approximately 500 meters (1,600 feet) below the surface. The team claimed these conditions are similar to DRGs with broken protective casings.

The team knew from previous work that most of the uranium minerals in the Kiggavik deposits are more than 274 million years old, but about 10% are less than 1 million years old. They found that these younger uranium minerals occurred alongside clay minerals surrounding the older uranium minerals and within open fractures. The researchers interpreted the older uranium as the source material of the younger uranium because of their proximity. They concluded that water must have infiltrated the deposits and interacted with older minerals, transporting uranium and forming new, younger minerals.

To determine the source of water infiltration, the researchers compared the chemistry of clay minerals formed alongside the younger uranium with modern snow in the Kiggavik region. They looked at the number of neutrons, or isotopes, in hydrogen atoms within the clay minerals, and found heavy values matching those of fresh water. These values indicated that snow or glacial meltwater was the source, rather than underlying bedrock or seawater. 

To determine when the water infiltrated over the last million years, the researchers used uranium-thorium dating to find when the radionuclides were mobilized. This dating method shows when uranium was removed from the original minerals by measuring how much its concentration was reduced by water infiltration. They found that the dates of these radionuclide mobilizations clustered into 5 separate water-infiltration events that occurred from 36 to 470 thousand years ago. 

Comparing these dates with existing climate records, the researchers found that the water infiltration events overlapped with periods when glaciers covered the ground surface. Normally, water can’t travel thousands of feet underground. But, like a hydraulic press, a glacier pushes water at its base downward. If the water encountered cracks in the ground, the pressure could inject it to the depth of the uranium deposit.

Next, the researchers wanted to measure how quickly radionuclides left the deposit. They measured the maximum distance between younger and older uranium minerals and used the age of the most recent water-infiltration event to estimate the fastest speed at which the radionuclides could have moved. They determined this maximum speed to be 0.0014 millimeters (0.000055 inches) per year. At that rate, they calculated it would take 360 million years for uranium radionuclides to travel 500 meters from a DGR at the same depth as the Kiggavik uranium deposits to the surface. This timescale far exceeds the necessary 1 million years for nuclear waste containment.

The team concluded that if glaciers were to return to Kiggavik, the uranium deposits wouldn’t experience substantial radionuclide movement, even in contact with glacial meltwater. Similarly, if a DGR has broken protective casings, radionuclides would be moving too slowly to reach the surface even if water leaked into it. They suggested their findings reduce constraints on viable DGR sites and could help ensure the long-term secure storage of nuclear waste.

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The post-World War II mining rush that transformed a region.

Fluxnium's fibers can extract uranium directly from seawater, which has enough of the nuclear fuel to power thousands of generations.

The company, which filed for bankruptcy protection in 2017, stands to benefit from growing support for nuclear power and President Trump’s deal with Saudi Arabia.

The company, which filed for bankruptcy protection in 2017, stands to benefit from growing support for nuclear power and President Trump’s deal with Saudi Arabia.

© Mike Stewart/Associated Press

In a groundbreaking development that could revolutionize the future of clean energy, researchers have unveiled a novel material designed to dramatically enhance uranium extraction from seawater. This latest advance hinges on the innovative use of a hollow hierarchical covalent organic framework (COF) characterized by bidirectional reactant flux coupling. This unique structural and functional design enables unprecedented efficiency in capturing uranium ions from the vast and dilute oceanic reservoir, with far-reaching implications for sustainable nuclear fuel supply and environmental safety.

Uranium, a critical element for nuclear power generation, exists in seawater at extremely low concentrations—approximately 3.3 parts per billion—making its extraction both technically challenging and economically prohibitive. Traditional mining techniques are resource-intensive and often environmentally detrimental, prompting scientists to pursue alternative methods, particularly adsorption-based techniques that leverage specialized materials. However, these materials have generally faced limitations including slow kinetics, low selectivity, and limited recyclability, which curb their practical applications. The newly reported COF-based material addresses these challenges head-on by integrating a sophisticated structural architecture with dynamic chemical properties.

At the heart of this innovation is the concept of bidirectional reactant flux coupling, an advanced mechanism that facilitates the simultaneous transport and interaction of multiple ionic species within the COF pores. Unlike conventional adsorbents that rely on unidirectional ion diffusion, this bidirectional flux allows for a synergistic interplay between uranium ions and reactants or functional groups embedded within the framework, effectively accelerating the uranium uptake process. This mechanism enhances both the selectivity and capacity of the material, setting a new benchmark in adsorption performance.

The hollow hierarchical structure of the COF is ingeniously engineered to maximize surface area while ensuring efficient mass transport. Typically, higher surface areas in porous materials improve adsorption capabilities, but this often comes at the expense of diffusion limitations. In this COF, the hierarchy of pore sizes—from large hollow channels to smaller microporous domains—facilitates rapid access of uranium ions to active binding sites. This multiscale porosity coupled with controlled chemical environments inside the pores promotes a highly effective capture process that is robust even in the complex ionic milieu of seawater.

Chemically, the COF is adorned with functional groups optimized to coordinate uranium ions selectively. These ligands, typically nitrogen- or oxygen-rich moieties, form strong yet reversible bonds with uranyl species (UO2^2+) prevalent in seawater. The reversibility of the adsorption process is crucial, enabling the material to be regenerated and reused multiple times without significant loss of performance. This recyclability translates into lower operational costs and improved environmental metrics compared to single-use adsorbents or conventional extraction methods.

The synthesis of the COF involves precise control over the polymerization conditions and monomer selection to achieve the desired hollow architecture and functional group distribution. The researchers utilized a bottom-up strategy, leveraging modular organic building blocks that self-assemble into crystalline frameworks. This level of control allows for fine-tuning the pore size distribution, surface chemistry, and overall material stability, ensuring resilience under harsh marine conditions. The material exhibits excellent chemical and mechanical stability, maintaining performance over extended exposure to seawater, which is a known challenge for many adsorbents due to corrosive salts and biofouling.

Advanced characterization techniques, such as electron microscopy and synchrotron X-ray diffraction, were employed to elucidate the hierarchical structure and confirm the integrity of the COF post-synthesis. Spectroscopic analyses, including X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR), provided insights into the chemical environment of the active sites and the nature of the uranium interaction. In situ studies revealed real-time adsorption dynamics, underpinning the understanding of the bidirectional flux and exchange mechanisms.

Benchmarked against existing adsorbents, the hollow hierarchical COF demonstrated uranium extraction efficiencies that are significantly higher, with faster kinetics and enhanced selectivity. The material consistently achieved high uranium uptake capacities in seawater samples, far exceeding traditional polymeric adsorbents and metal-organic frameworks previously reported. Equally important, the COF retained its adsorption performance over numerous cycles, underscoring its practical potential for long-term deployment.

The implications of this research extend beyond uranium extraction alone. The fundamental principles of bidirectional reactant flux coupling and hierarchical structural design can be applied to other separation and purification challenges in aqueous environments—such as desalination, pollutant removal, and rare earth element recovery. This cross-disciplinary potential suggests that the material design approach could ignite a new era in functional porous materials tailored for sustainability.

Furthermore, by enabling access to one of the planet’s most abundant uranium reservoirs, the oceans, this technology promises to mitigate supply risks associated with terrestrial uranium mining. Given the strategic importance of nuclear energy in reducing carbon emissions and combating climate change, materials that make nuclear fuel sourcing cleaner and more efficient could accelerate the transition to low-carbon energy systems globally.

Environmental considerations were also addressed, with preliminary life cycle analyses indicating that using the COF-based extraction process can reduce the ecological footprint relative to conventional mining. The absence of harmful solvents and the material’s recyclability contribute to eco-friendliness, while the ability to capture uranium directly from seawater minimizes disruption to terrestrial ecosystems. The potential for integrating this material into large-scale ocean-based extraction systems holds promise for sustainable resource utilization.

Looking ahead, the challenges to commercialization include scaling synthesis, engineering large-format adsorbent modules, and optimizing deployment strategies in marine environments. However, the foundational knowledge established in this study provides a clear roadmap for overcoming these hurdles. Collaborative efforts between material scientists, chemical engineers, and environmental specialists will be crucial to translate this laboratory breakthrough into a viable industrial technology.

The research team envisions that future iterations of the COF will incorporate multifunctional sites to simultaneously capture other critical elements present in seawater, thereby enhancing economic viability. Moreover, integration with renewable energy-powered extraction stations could further reduce the carbon footprint of nuclear fuel procurement, aligning with global clean energy initiatives.

In summary, this pioneering work represents a significant stride toward unlocking the ocean’s uranium wealth through innovative material science. The hollow hierarchical covalent organic framework with bidirectional reactant flux coupling introduces a new paradigm in adsorption technologies, combining structural elegance with chemical sophistication. As the world grapples with energy security and environmental responsibility, such advances offer a beacon of hope and a tangible path forward for sustainable nuclear energy production.

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Subject of Research: Uranium extraction from seawater using advanced covalent organic frameworks.

Article Title: Bidirectional reactant flux coupling in hollow hierarchical covalent organic framework enabling efficient uranium extraction from seawater.

Article References:
Zhang, J., Cao, X., Zhang, J. et al. Bidirectional reactant flux coupling in hollow hierarchical covalent organic framework enabling efficient uranium extraction from seawater. Nat Commun (2026). https://doi.org/10.1038/s41467-026-74724-3

Image Credits: AI Generated

Companies say it’s a better way to dispose of the Cold War-era material — and fix a shortage of nuclear fuel. But the plan has also faced criticism from nonproliferation experts.

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